It is a good idea to have a regular backup of your hosted website. Your hosting company should have backup capability as part of your package. I would check out what they have available most likely it is on the C-Panel.
As to your surprise. I wasn't there to see what you did, so I'm not sure if you were accessing your HTML saved filed on your own local computer and surfing your website on your local computer from the saved HTML or possibly you were viewing cached pages, again from your local computer.
As to your question. If your website host goes completely dark the only thing you would be able to see are cached pages. And they won't be coming from your non-functioning host server.

I would try resetting the swap file to clear it out. Manage it and set it to an arbitrary number like 500mb and restart then put it back the way you had it and restart.
I would also clear out temp files, perhaps use TFC by Old Timer or another decent tool for it.
I would take a hard look at running programs. I've used Autoruns by Systeminternals for this in the past.
Lacking a software solution, your HDD could be on its way out and having performance issues before it goes out the door.

That looks and sounds unpleasant and painful.
It is good to know and see that your well enough afterward to do the show and tell.
I suspect the most painful days from it are now upon you.
God Bless You!

Your system sounds top notch to me. I can't think of any issues that would be found there. (I'm making the assumption that your mobo drivers are up to date).
As you may already be thinking, running the test back to back with some high speeds and low speeds on the download side would seem to indicate the problem is farther out than Comcast. Maybe the route your location takes to bring some things down has a bottleneck that you won't be able to do anything about. I think if after several spaced out tests, if certain download test locations have a consistent high with the others consistent low that would confirm a problem outside of what you can influence.

Seems like your SSD's should perform without slowing things down. Have you looked at your CPU activity when the slowdowns happens? Maybe a clue will show with that info.
But, from what you've said so far I'm leaning toward something going on with COMCAST, such as daily peak activity periods which they are not capable of keeping up with.

First thing I would do, turn off anti-virus and any other active on the fly protection that you have running and run the test a few times again without that on. In my mind that is the most probable culprit.
After that I would look at any of that "gee wiz this sure makes my desktop look pretty stuff" visual affects, and turn it all off for the test. This Website discusses the built in affects and recommends what to turn off if you don't want to simply adjust for performance. http://www.thewindowsclub.com/disable-visual-effects-windows Be sure to only visit this and other websites with your anti-virus and any other protection you have installed turned on.
If you have installed any desktop features on top of what Win10 already has, you may want to disable them temporarily.
I also turn off the Indexing service for windows search which uses HDD and CPU resources.
There's probably some other things to look at but I don't remember them off the top of my head. You can find them by searching google for "tweak Win10". The things that make your performance faster by tweaking will also, most likely, affect your internet speed.
There are some things you cannot control out there in internet land that can also affect the speed. You can discuss this with your internet provider and see if they have any default server options in place which are affecting the speed that you can request to use or not use. Of course, they may try to upsell you which is always a risk when talking to them.
Oh, and Merry Christmas

Yes, if you change your password during the I Lost My Password reset process you will need to login using the new password on any other devices that uses the same service. You may or may not be forced to do it right away. If you shut a device off and turn it back on, or otherwise reset its connection, it will require the new password at that point.